RFR (XS): 8145674: Fix includes and forward declarations in g1Remset files

[Thomas Schatzl]

Hi all,

  can I have reviews for this change that adds necessary includes and
forward declarations in g1RemSet.?pp files? While it compiles right now
as is, if something changes in this area, compilation tends to break in
interesting ways.

CR:
https://bugs.openjdk.java.net/browse/JDK-8145674
Webrev:
http://cr.openjdk.java.net/~tschatzl/8145674/webrev/
Testing:
jprt

Thanks,
  Thomas